BUG/MEDIUM: mworker/cli: crash when trying to access an old PID in prompt mode

BUG/MEDIUM: mworker/cli: crash when trying to access an old PID in prompt mode

The master process encounter a crash when trying to access an old
process which left from the master CLI.

To reproduce the problem, you need a prompt to a previous worker, then
wait for this worker to leave, once it left launch a command from this
prompt. The s->target is then filled with a NULL which is dereferenced
when trying to connect().

This patch fixes the problem by checking if s->target is NULL.

Must be backported as far as 2.0.

(cherry picked from commit dcbe7b91d69f6857961d1545ae71205d9afb905f)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>